Cargo sorting at the trans-Golgi network at a glance

ABSTRACT The Golgi functions principally in the biogenesis and trafficking of glycoproteins and lipids. It is compartmentalized into multiple flattened adherent membrane sacs termed cisternae, which each contain a distinct repertoire of resident proteins, principally enzymes that modify newly synthesized proteins and lipids sequentially as they traffic through the stack of Golgi cisternae. Upon reaching the final compartments of the Golgi, the trans cisterna and trans-Golgi network (TGN), processed glycoproteins and lipids are packaged into coated and non-coated transport carriers derived from the trans Golgi and TGN. The cargoes of clathrin-coated vesicles are chiefly residents of endo-lysosomal organelles, while uncoated carriers ferry cargo to the cell surface. There are outstanding questions regarding the mechanisms of protein and lipid sorting within the Golgi for export to different organelles. Nonetheless, conceptual advances have begun to define the key molecular features of cargo clients and the mechanisms underlying their sorting into distinct export pathways, which we have collated in this Cell Science at a Glance article and the accompanying poster.

Lipid Profiles of RAS Nanoclusters Regulate RAS Function

The lipid-anchored RAS (Rat sarcoma) small GTPases (guanosine triphosphate hydrolases) are highly prevalent in human cancer. Traditional strategies of targeting the enzymatic activities of RAS have been shown to be difficult. Alternatively, RAS function and pathology are mostly restricted to nanoclusters on the plasma membrane (PM). Lipids are important structural components of these signaling platforms on the PM. However, how RAS nanoclusters selectively enrich distinct lipids in the PM, how different lipids contribute to RAS signaling and oncogenesis and whether the selective lipid sorting of RAS nanoclusters can be targeted have not been well-understood. Latest advances in quantitative super-resolution imaging and molecular dynamic simulations have allowed detailed characterization RAS/lipid interactions. In this review, we discuss the latest findings on the select lipid composition (with headgroup and acyl chain specificities) within RAS nanoclusters, the specific mechanisms for the select lipid sorting of RAS nanoclusters on the PM and how perturbing lipid compositions within RAS nanoclusters impacts RAS function and pathology. We also describe different strategies of manipulating lipid composition within RAS nanoclusters on the PM.

Caveolin-1 and cavin1 act synergistically to generate a unique lipid environment in caveolae

Caveolae are specialized domains of the vertebrate cell surface with a well-defined morphology and crucial roles in cell migration and mechanoprotection. Unique compositions of proteins and lipids determine membrane architectures. The precise caveolar lipid profile and the roles of the major caveolar structural proteins, caveolins and cavins, in selectively sorting lipids have not been defined. Here, we used quantitative nanoscale lipid mapping together with molecular dynamic simulations to define the caveolar lipid profile. We show that caveolin-1 (CAV1) and cavin1 individually sort distinct plasma membrane lipids. Intact caveolar structures composed of both CAV1 and cavin1 further generate a unique lipid nano-environment. The caveolar lipid sorting capability includes selectivities for lipid headgroups and acyl chains. Because lipid headgroup metabolism and acyl chain remodeling are tightly regulated, this selective lipid sorting may allow caveolae to act as transit hubs to direct communications among lipid metabolism, vesicular trafficking, and signaling.

Dissecting the nanoscale lipid profile of caveolae

Caveolae are specialized domains of the vertebrate cell surface with a well-defined morphology and crucial roles in cell migration and mechanoprotection. Unique compositions of proteins and lipids determine membrane architectures. The precise caveolar lipid profile and the roles of the major caveolar structural proteins, caveolins and cavins, in selectively sorting lipids have not been defined. Here we used quantitative nanoscale lipid mapping together with molecular dynamic simulations to define the caveolar lipid profile. We show that caveolin1 (CAV1) and cavin1 individually sort distinct plasma membrane lipids. Intact caveolar structures composed of both CAV1 and cavin1 further generate a unique lipid nano-environment. The caveolar lipid sorting capability includes selectivities for lipid headgroups and acyl chains. Because lipid headgroup metabolism and acyl chain remodelling are tightly regulated, this selective lipid sorting may allow caveolae to act as transit hubs to direct communications among lipid metabolism, vesicular trafficking and signalling.

Force Barrier for Lipid Sorting in the Formation of Membrane Nanotubes

Understanding lipid sorting of multicomponent cell membranes associated with tubular deformation is of essential importance to many cell activities such as filopodial growth and protein-mediated vesiculation. Here, we conduct theoretical analysis to investigate how the membrane tubulation induced by an external pulling force over a finite region is regulated by the coupling between the lipid composition and the membrane bending rigidity and tension. It is shown that the presence of the lipid-disordered phase facilitates the nanotube formation by reducing the force barrier. As the pulling region size and the membrane tension increase, the membrane tubulation becomes discontinuous regardless of the coupling effect. The direct proportional relationships between the maximum pulling force and size of pulling region at different coupling scenarios are identified. Analytical solutions for the linear force-extraction relation and the membrane configurations in the early stage of the membrane extraction are obtained. Our results indicate that in the case of a relatively small pulling region, the coupling between the membrane composition and mechanical properties plays an important role in regulating the membrane extraction, and such an effect due to the phase separation diminishes gradually as the pulling region enlarges and the force barrier becomes dominated by a large pulling region.

Membrane curvature sensing of the lipid-anchored K-Ras small GTPases

Cell morphologies, defined by plasma membrane (PM) local curvature, change during mitogen-dependent function and pathology, such as growth, division and proliferation. The lipid-anchored Ras small GTPases are essential upstream regulators of the mitogen-activated protein kinases (MAPKs) cascades and play key roles in many pathological conditions, especially cancer. Ras signaling is mostly compartmentalized to the cell PM through the formation of nanometer-sized domains, termed as nanoclusters, and undergo selective lipid sorting for efficient effector recruitment and activation. Thus, Ras function might be sensitive to changing PM curvature, potentially regulating mechanosensing of mitogen signaling. We employed nanofabrication and super-resolution imaging and found that Ras functions respond to PM curvature modulations in an isoform specific manner: nanoclustering and signaling of the most oncogenically prevalent isoform K-Ras favor less curved PM, while those of another isoform H-Ras favor more curved PM. We then examined whether Ras membrane curvature sensing is mediated by lipid sorting. We found that anionic phospholipids sense changing PM curvature in distinct manners: phosphatidylserine (PS) localization shows preference for less curved membrane but phosphoinositol 4,5-bisphosphate (PIP2) localization favors more curved PM. Depletion of endogenous PS abolishes K-Ras PM curvature sensing. Exogenous PS addback and synthetic bilayer binding assays further show that only mixed-chain PS species, but not other PS species, mediate K-Ras curvature sensing. Taken together, the Ras proteolipid nano-assemblies on the PM act as relay stations to convert mechanical stimulations to mitogenic signaling circuits, thus a novel mechanism for cancer cell mechanotransduction.